Alkylation of alkyl aromatic hydrocarbons over metal oxide-alkali metal superbasic catalysts

Article abstract of DOI:10.1006/jcat.2001.3327

The alkylation of toluene, ethylbenzene, cumene, and o-, m-, and p-xylenes with ethylene, propylene, and 1,2-diphenylethylene was studied over superbasic MgO-K and γ-Al₂O₃-K catalysts and over model systems of the electron donor acceptor complex type. The ethylation and propylation of alkylbenzenes indicated that the donor power and the concentration of the one-electron donor centers were not the only factors, which determined the activity (depicted by the initial reaction rate, turnover number, or alkylbenzene conversion) and selectivity of the catalytic system. In the series of reactions, a higher total conversion of alkyl aromatic hydrocarbons to their ethylation or propylation products was achieved over γ-Al₂O₃-K systems. The reaction chemoselectivity (mono- or difunctionalization of alkylbenzenes) depended on the nature of the alkyl aromatic reactant and alkylating alkene, on the reaction temperature, and on the used catalyst.

Full text of DOI:10.1006/jcat.2001.3327

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Journal of Catalysis 203, 407–425 (2001)
doi:10.1006/jcat.2001.3327, available online at http://www.idealibrary.com on
Alkylation of Alkyl Aromatic Hydrocarbons over Metal Oxide–Alkali
Metal Superbasic Catalysts
J. Kijen´ski,1 P. Radomski, and E. Fedoryn´ska
Faculty of Chemistry, Warsaw University of Technology (Politechnika), ul. Noakowskiego 3, 00-664 Warsaw, Poland
Received March 19, 2001; revised June 25, 2001; accepted July 2, 2001
–
–
Ph CH2CH2CH a + Ph CH
2N
3
The alkylation of toluene, ethylbenzene, cumene, and xylenes
with ethene, propene, and 1,2-diphenylethylene was investigated
over superbasic MgO–K and -Al2O3–K catalysts and over model
systems of the EDA complex type. Metal oxide–alkali metal solid
superbases as well as MgO–K polyaromatic hydrocarbon systems
exhibited high activity in the transformations carried out under at-
mospheric and elevated pressure at low temperature. The reaction
kinetics, the results of ESR investigations of adsorbed intermedi-
ates, and the effects of poisoning using radicals trap TEMPO en-
abled the identification of F+s sites as catalytically active centres.
The proposal of a radical pathway of the reaction was confirmed
c
*
)
–
–
Ph CH CH CH + Ph CH
a
2N
[3]
2
2
3
The same mechanism was adopted by workers in Japan
(14) for the alkylation of alkylbenzenes with alkenes over
the complex solid three-component superbasic catalyst
Al2O3–NaOH–Na.
The resultsofour studies(1979–1998) on solid superbases
of the metal oxide–alkali metal type demonstrated the high
activity of superbasic systems in the selective hydrogena-
tion of various alkenes (e.g., ethene, propene, cyclohexene,
styrene, and isoprene) and carbon monoxide (15–22). Our
superbases are bifunctional systems with extremely strong
-
and illustrated.
2001 Academic Press
Key Words: alkylbenzenes side-chain alkylation; superbasic
catalysts; one-electron donor centre activity; radical reaction path-
way.
two-electron donors (Hammett basic strength, H
37)
and one-electron donor (able to ionize molecules with
electron affinity values in the range of 0.2 eV) centres
on their surface. Even so, it was unequivocally proved by
the catalytic poisoning and by the correlation activity vs the
concentration of the one-electron donor centres that the
extremely strong single-electron donor sites (surface Fs+
centers—electrons from alkali atoms in anionic vacancies
of metal oxides) were responsible for the initiation of the
above-mentioned hydrogenation reactions (17). The sec-
ondary surface centres of an anion radical type originated
from alkenes or CO adsorption on F+s sites were postu-
lated to play the decisive role in the activation of the dihy-
drogen molecule (19–22). Only recently, the importance of
secondary radicals was confirmed by the modelling of sur-
face anion radical centres of differentiated power through
the adsorption of various polyaromatics onto F+s centres of
the superbasic catalyst -Al2O3–K or by the immobiliza-
tion of the ion radical salt (potassium naphthenide) on the
-Al2O3 surface (21).
INTRODUCTION
Only few methods have been proposed for the side-
chain alkylation in alkylbenzenes (1–12). The most effec-
tive method was the reaction of benzene alkyl derivatives
with alkenes as alkylating agents in the presence of ex-
tremely strong basic catalysts, largely developed by Pines
and co-worker (1, 2). As highly basic catalyst alkali metals,
their hydrides and graphite intercalates were used. Espe-
cially active catalytic systems were sodium and potassium
organic complexes with, e.g., polyaromatic hydrocarbons,
alkylchlorides, benzonitrile, or pyridine (1, 2). All these
catalysts were assumed to transform alkylbenzenes to ben-
zylic carboanions via a transmetalation reaction. Therefore,
the reaction mechanism proposed by Pines and co-worker
(13) involves the addition of a benzylic carboanion to the
alkene molecule:
The majority of the basic catalysts used by Pines and
[1] co-worker, (1, 2) for side-chain alkylation in alkylbenzenes
were EDA (electron donor acceptor) complexes or organo
–
–
Ph CH3 + B Na → Ph CH a + BH
2N
*
)
–
–
==
Ph CH a + H C CH2
2N
Ph CH CH CH
a
2N
[2]
2
2
2
alkali metal reagents, which can produce radicals, according
to Morton and co-workers (23, 24). Our superbasic cata-
lysts of the metal oxide–alkali metal type are also the EDA
complexes in which electron-deficient surface sites (anionic
1 To whom correspondence should be addressed. Fax: +48 22 621-37-20.
E-mail: jacekk@ch.pw.edu.pl.
407
0021-9517/01 $35.00
c
Copyright
2001 by Academic Press
All rights of reproduction in any form reserved.

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´ ´
KIJENSKI, RADOMSKI, AND FEDORYNSKA
408
vacancies (21, 22)) play the role of acceptors for alkali- zene (Aldrich 99% , dest. bp 408–409 K and dried or o-,
metal-derived electrons (the rule of F+s centres formation). p-, m-xylenes (Aldrich, >99)) were introduced into a glass
Furthermore, reactants such as alkenes or alkylbenzenes ampoule (20 cm3) that had been evacuated and rinsed with
form with F+s centres secondary EDA complexes which are dry-deoxygenated N2. The frozen ampoule was sealed and
of high cata- lytic importance for hydrogenation (of, e.g., placed into the evacuated and pure dinitrogen-washed au-
alkenes) or dehydrogenation (of, e.g., alkylaromatic hydro- toclave. The reactor was then rinsed with ethene or propene
carbons) reactions (21, 22, 26). Finally, the above reactions and filled with a given gas to 1.01 MPa (Aldrich, 99.5+% ,
proceed through radical intermediates. The listed similari- ethene) or 0.71 MPa (Aldrich, 99.5+% , propene). The au-
ties between the catalytic systems used by Pines and solid toclave was thermostatured, the ampoule with catalyst and
superbases prompted us to investigate the catalytic activity alkylbenzene was broken, and the remaining mixture was
of superbasic catalysts in the side-chain alkylation of alkyl stirred. The reaction products were analyzed by GC using a
derivatives of benzene. This work was also intended to be a 5-m column (OV 101/Chromosorb W) and detected by MS
study of the reaction mechanism. Alkyl aromatics cumene, (HP 4890 series II).
ethylbenzene, toluene, and o-, m-, and p-xylenes were cho-
The reactions under normal pressure were carried out
sen as a model; the alkylating agents were ethene, propene, according to a similar procedure in a glass batch reactor
and trans stilbene (1,2-diphenylethene). Excellent demon- (150 cm3) under normal pressure. In this case, 1.64 mmol of
strations of effects of the appearance of the radical species corresponding alkylbenzene (0.156 g of toluene, 0.174 g of
were achieved by means of the catalyst poisoning and the ethylbenzene, 0.196 g of cumene, or 0.177 g of correspond-
ESR measurements of the adsorbed states of the reactants. ing xylene) reacted with an excess of ethene or propene
The latter is a special focus of the work of this laboratory.
over 0.05 g of superbasic catalyst (MgO–K, -Al2O3–K, or
MgO–K–polyaromatic hydrocarbon).
The reactions with ethylene oxide were carried out
in a glass batch reactor at 373 K under normal pres-
sure; 1.64 mmol of corresponding alkylbenzene (0.156 g of
toluene, 0.174gofethylbenzene, and 0.196gofcumene) was
introduced into the evacuated and then dry-deoxygenated
dinitrogen-washed reactor and filled with ethylene oxide
(Fluka, purum, >99.8% ).
EXPERIMENTAL
Catalysts
Magnesia was obtained by hydrolysis of Mg(NO3)2
6H2O with a concentrated aqueous ammonia solution.
After the solution was washed with doubly distilled water,
the Mg(OH)2 precipitate was dried at 333, 353, and 393 K
for 24 h at each temperature.
-Alumina was prepared by hydrolysis of aluminium tri-
isopropoxide (Fluka, pract. dest. 98% Al), previously pu-
rified by distillation under vacuum (bp 413 K, 1.07 kPa).
The hydrolysis procedure has been described elsewhere
(27, 28). The obtained Al(OH)3 was dried at 313, 353, and
393 K for 24 h at each temperature.
Both hydroxides were calcined before potassium evapo-
ration, first in air at 823 K for 1 h and then in a stream of
water-free and deoxygenated nitrogen at the same temper-
ature for 16 h in the case of MgO and for 5 h in the case of
-Al2O3.
Metallic potassium (Fluka, purum, 98% ) was deposited
onto magnesia and -alumina surfaces by chemical vapour
deposition evaporation under reduced pressure (1.3 Pa) at
573 K. After evaporation the catalysts were evacuated at
623 K and then cooled in a stream of water-free deoxy-
genated nitrogen.
Poisoning of Active Sites
Reactions in the presence of the free radical trap
TEMPO (2,2,6,6-tetramethylpiperydynyl-1-oksyl, Aldrich,
99% ) were performed at 373 K for 5 h in the glass batch
reactor (150 cm3) with the same amounts of reactants as
those in reactions under normal pressure described above.
Then, 0.001 g of TEMPO was introduced into the reactor
together with 0.05 g of a catalyst.
Textural Properties of Studied Catalysts
The specific surface area and pore distribution on the sur-
faces of the studied catalysts were determined by BET using
a Quantasorb IR2 apparatus (Quantachrom) (Table 1).
TABLE 1
Specific Surface Area and Predominant Pore Radius
of the Studied Carriers and Superbasic Catalysts
Reactions
SBET
Pore radius
a
The alkylation reactions under pressure were performed
in a stainless steel pressure reactor (250 cm3) according to
the following procedure: 0.25 g of catalyst and 8.2 mmol
of alkylbenzene (0.78 g for toluene (Aldrich 99% , dest.
bp 283–384 K and dried), 0.98 g for cumene (Aldrich 99% ,
dest. bp 425–427 K and dried), and 0.87 g for ethylben-
1
Catalyst
MgO
MgO–K
-Al2O3
(m2g
)
A
37
31
92
90
74
71
38
37
-Al2O3–K

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ALKYLATION OF ALKYL AROMATIC HYDROCARBONS
409
ESR Measurements
tion of alkyl aromatics, however, gave a higher yield over
-Al2O3–K.
The ESR spectra were registered at room temperature
using a Brooker ESP 300E spectrometer. Alkylbenzenes
and alkenes (1.33 10 Pa) were adsorbed at room tem-
The only products formed in the ethylation or propy-
lation of the studied alkylbenzenes under alkylation
conditions were their respective mono-, di-, or trialkyl
derivatives. The amount of the formed unidentified high-
temperature boiling fraction did not exceed 0.1% . No
oligomerization products of the alkylating alkenes formed
in the reactant solution (liquid phase) or in the gas phase.
Only in the case of reactions of alkyl aromatics, carried out
with ethene at the highest reaction temperature (423 K), did
the surface of the catalyst turn the characteristic yellow. IR
spectra of the outgassed catalyst samples (1.03 Pa) revealed
3
perature on the surface of the catalysts previously evacu-
ated to 1.33 10 2 Pa.
RESULTS
Alkylation of Toluene, Ethylbenzene, and Cumene
Pure magnesia and -alumina were inactive in the stud-
ied transformations. Both investigated superbasic catalysts,
the MgO–K and the -Al2O3–K system, exhibited high
activity in all the ethylation and propylation reactions of
monoalkylbenzenes (Tables 2 and 3). The MgO–K cata-
lyst was found to be considerably more active than the
-Al2O3–K system in the ethylation of toluene, ethylben-
zene, and cumene under elevated pressure. The propyla-
1
the presence of characteristic bands at 2935 to 2830 cm
1
and 1480 to 1430 cm attributed by us (21) to the formed
polymeric residue. Such a residue did not form in reactions
performed at lower temperature ( 373 K) and was there-
fore not observed.
It is noteworthy that the catalysts used at tempera-
tures 373 K maintained their activity after the reaction
TABLE 2
Reaction Products in the Alkylation of Aromatics with Ethene and Propene [Reaction Time: 5 h (Ethene), 10 h (Propene); Pressure:
1.01 MPa (Ethene), 0.71 MPa (Propene)]
Product (mol% )
Ethylation
Propylation
Monoalkyl
derivativea
Dialkyl
Monoalkyl
derivatived
Dialkyl
Reactant
Toluene
Catalyst
MgO–K
T (K)
derivativeb
Indanec
derivativee
Indane f
323
373
423
323
373
423
323
373
423
473
323
373
423
473
323
373
423
323
373
423
2.6
26.8
34.6
0.5
0.7
0.8
33.0
39.6
48.7
41.5
21.2
23.3
32.6
40.0
75.8
77.7
89.8
69.5
48.3
39.6
0.0
3.0
1.4
0.0
0.0
0.0
1.8
4.4
0.6
10.1
6.1
3.8
5.6
4.6
4.3
7.6
7.4
4.0
1.7
0.0
0.0
0.0
0.0
0.0
0.0
0.0
2.0
0.0
0.0
2.5
0.0
0.0
1.0
3.5
0.0
0.0
0.0
0.3
0.0
0.0
5.4
5.9
23.0
1.2
7.0
37.4
3.2
33.0
51.0
60.0
7.2
48.8
45.7
53.4
0.0
0.0
0.0
2.4
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
1.1
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.4
0.3
0.6
0.0
0.6
0.3
0.0
0.0
0.0
-Al2O3–K
MgO–K
Ethylbenzene
-Al2O3–K
Cumene
MgO–K
5.3
10.0
0.0
10.6
16.7
-Al2O3–K
a Monoalkyl derivative of toluene: 1-phenylpropane; of ethylbenzene: 2-phenylbutane and traces of 1-phenylbutane; of cumene: 2-methyl-2-
phenylbutane and traces of 3-methyl-3-phenylhexane.
b Dialkyl derivative of toluene: 3-phenylpentene; of ethylbenzene: 3-methyl-3-phenylpentane; of cumene: 3-methyl-3-phenylhexane.
c Indane: 1-methylindane for ethylbenzene ethylation; 1,2-dimethylindane for cumene ethylation.
d Monoalkyl derivative of toluene: 2-methyl-1-phenylpropane and traces of 1-phenylbutane; of ethylbenzene: 2-methyl-3-phenylbutane and traces
of 2-phenylpentane; of cumene: 2,3-dimethyl-2-phenylbutane and traces of 2-methyl-2-phenylpentane.
e Dialkyl derivative of toluene: 3-phenylpentene; of ethylbenzene: 3-methyl-3-phenylpentane; of cumene: 3-methyl-3-phenylhexane.
f
Indane: 1,2-dimethylindane for ethylbenzene propylation; 1,1,2-trimethylindane for cumene propylation.

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410
´
´
KIJENSKI, RADOMSKI, AND FEDORYNSKA
TABLE 3
Reaction Products in the Alkylation of Equimolar Mixtures of Alkyl Aromatics over MgO–K Catalyst
[Reaction Time: 5 h, Temperature: 373 K, Normal Pressure]
Starting
composition
Composition of reagents after reaction (mol% )
(mol% )
Toluene
46.2
Ethylbenzene
33.8
Cumene
—
A
B
A/B
0.18
Toluene–Ethylbenzene
Toluene–Cumene
49.3
50.7
3.1
16.9
48.7
51.3
45.2
—
—
25.1
35.4
3.5
26.2
16.1
0.13
0.80
Ethylbenzene–Cumene
48.5
51.5
35.6
12.9
Note. A, monoalkyl derivative of hydrocarbon possessing benzyl carbon atom of lower order. B, monoalkyl
derivative of hydrocarbon possessing benzyl carbon atom of higher order. Monoalkyl derivatives: 1-phenylpropane
for toluene, 2-phenylpentane for ethylbenzene, and 2-methyl-2-phenylbutane for cumene.
run. In the event of subsequent use of the catalysts, while
maintaining the water and oxygen-free conditions of all the
operations, the decrease in the rate of a particular reaction
did not exceed 12 to 15% .
The reaction of toluene with ethene proceeded accord-
ing to Eq. [4] with the formation of the mono- and dialkyl
derivatives, 1-phenylpropane (II) or 3-phenylpentane (III):
[6]
Ethylbenzene propylation led to 2-phenylpentane
(XII), 2-methyl-3-phenylbutane (XIII), 2,3,4-trimethyl-3-
phenylpentane (XIV), and 1,2-dimethylindane (XV):
[4]
The products of toluene alkylation with propene were
2-methyl-1-phenylpropane (IV), 1-phenylbutane (V), and
2,4-dimethyl-3-phenylpentane (VI):
[5]
[7]
During ethylbenzene ethylation, monoalkyl derivatives
The products of cumene alkylation with ethene were a
(2-phenylbutane (VIII) and traces of 1-phenylbutane (IX)) monoalkyl derivative (2-methyl-2-phenylbutane (XVII)), a
and a dialkyl derivative (3-methyl-3-phenylpentane (X)) dialkyl derivative (3-methyl-3-phenylhexane (XVIII)), and
were accompanied by 1-methylindane (XI):
a cyclization product (1,1-dimethylindane (XX)):